1. Field of the Invention
The invention contained herein pertains to the field of gas chromatography and more specifically to compact capillary column assemblies and the thermal modulators used to modulate the temperature of the compact capillary column assemblies in order for chemical compounds to be optimally separated from one another. The invention is primarily concerned with thermally efficient, compact, dimensionally flat, rugged, low thermal mass capillary column assemblies of varying flat geometries and the attachment of such capillary column assemblies to various types of thermal modulators for integration with a wide variety of currently available gas chromatographic devices. It is the ultimate goal of this invention to provide the field of gas chromatography with a device that is extremely compact, consumes the smallest amount of power possible, can be efficiently and accurately reproduced and yet can still extract the maximum amount of theoretical separation efficiency from the capillary column material.
2. Prior Art
In the field of gas chromatography a chemical sample is physically placed into the injection port of a gas chromatograph (GC). The injection port is usually at an elevated temperature such that the chemical sample, if not already in the gas phase, becomes immediately vaporized. A stream of continuously flowing gas, referred to in the field as carrier gas (mobile phase), sweeps the vaporized chemical sample into a chromatographic separation column, which, in most state-of-the-art high resolution chromatographic devices, consists of a hollow tube that can vary in length from mere tens of centimeters to hundreds of meters in length. The inside diameter of such tubes can also range in size from 25 micrometers up to 530 micrometers and are generally produced from fused silica, coated with a layer of high temperature polyimide or from long sections of metal capillary tubing. Evenly coated on the inner surface of the separation column is a thin layer of viscous or polymeric material with specific chemical properties that are chosen to interact with the chemical sample previously swept inside by the carrier gas. This type of separation column is generally referred to as a capillary chromatographic separation column or capillary column for short and can be obtained in bulk lengths from several different manufacturers such as Agilent Technologies, Varian/Chrompack, Restek, Supelco, SGE, VICI and Quadrex.
The “heart” of any chromatographic device is its separation column. The separation column operates through the interaction of the thin layer of material coated on its inner surface and the vaporized chemical sample placed inside one end of the separation column by the carrier gas. As the carrier gas and chemical sample progress down the length of the separation column, the individual chemical compounds contained in the sample interact at different rates with the coating on the inner surface of the tube. These interactions between the different compounds contained in the chemical sample and the inner surface of the tube effectively reduces the speed at which each compound traverses the length of the separation column. Dependent upon to what degree each chemical compound interacts with the material coating on the inner surface of the separation column, the overall effect is that each chemical compound becomes spaced in time from one another and ultimately exits the end of the separation column, opposite the injection end, in discrete “bands” ideally containing only one chemical compound. The discrete “bands” exiting the separation column are then channeled to a detection device which determines at frequent, regular intervals of time, how much chemical sample is present and with certain classes of detection devices, what type of chemical is present also.
The amount of time required for a chemical compound to traverse the entire length of a separation column is known as its retention time. In high performance capillary gas chromatography four major factors contribute to the retention times of chemical compounds: separation column length, chemical composition of the coating on the inner surface of the separation column (stationary phase), linear carrier gas velocity, and most significantly, the temperature of the separation column. Well known in the field is the fact that for a given separation column length and stationary phase coating, there exists an optimal linear carrier gas velocity and separation column temperature where individual chemical compounds pass through the separation column and are separated from similar chemical compounds most efficiently. This optimal set of conditions is only effective for a narrow range of chemical compounds and any compounds that exist outside of this narrow range may not separate from one another or may exit from the separation column very slowly if at all. This proved to be very limiting if a chemical sample to be analyzed contained compounds with a broad boiling point distribution. Fortunately, it was discovered that by increasing the temperature of the separation column while the chemical compounds were traveling down the length of the separation column, in effect, the optimal conditions “window” could be moved in time such that at least a portion of the time each chemical compound spent in the separation column was under optimal conditions. This discovery greatly increased the range of chemical compounds that could be efficiently separated in a single analysis cycle and quickly expanded the use of gas chromatographs.
The method of changing the temperature of the separation column while the chemical sample progresses down the length of the separation column is referred to as temperature programmed gas chromatography. The faster the separation column is temperature programmed, the faster a single analysis cycle can be completed thus increasing the number of chemical samples that can be analyzed in a given length of time. In order to increase the temperature of the separation column in a precise manner the chromatographic device must contain a controlled thermal region to house the separation column. In conventional gas chromatographs this is accomplished by incorporating a convection oven large enough to house the capillary separation column which is usually coiled on a round wire rack approximately six inches in diameter. The wire rack containing the separation column hangs freely in the oven with the open ends connected to the injector and detector ports which protrude through the walls of the oven box. The air contained inside the oven is heated by wire filaments and is circulated with a fan located inside the oven in order to maintain a constant, even temperature distribution throughout the entire oven volume. Thermal energy from the heated air is then used to heat the relatively insignificant thermal mass of the separation column. This results in very even heating of the separation column along its entire length and consequently produces very efficient chromatographic performance, but at a significant cost. The amount of power required to heat the oven volume to a normal upper temperature limit of approximately 350 to 400 degrees C. is on the order of single digit kilowatts with a heating rate limit usually around 90 degrees C./minute, when in contrast, the amount of power required to heat the separation column material only is orders of magnitude less. Much energy is wasted. Because of the large amount of power being consumed to heat the oven, large amounts of insulation must be employed to contain the heat, consequently, the bulky nature of conventional gas chromatographs can be directly attributed to the oven/insulation combination. Due to the large amount of power and space required to thermally modulate and house the capillary separation column when coiled on a conventional wire rack, this design is impractical for use in miniaturized or portable/transportable gas chromatographic systems. Several successful attempts have been made at reducing the separation column wire rack size and the ovens that contain them. While power consumption was reduced, these gas chromatographs still consume relatively large amounts of power, on the order of a kilowatt, and are mostly confined to isothermal oven operation or very slow temperature programming rates with limited upper temperatures when being used in a transportable mode where power supply is a relevant issue. This severely limits the range of compounds that can be analyzed as well as increases the amount of time necessary for each analytical cycle. It became quickly apparent that if temperature programmed gas chromatographs were to become smaller, faster and less demanding of power, which are desired traits in nearly all cases, a more energy efficient method of thermally modulating the separation column was going to be needed.
One solution to this problem is to transfer thermal energy directly to the surface of the separation column rather than heating the air surrounding it and having the air transfer the thermal energy. Sides, et al. in U.S. Pat. No. 5,014,541 describe a miniaturized separation column assembly that replaces the traditional column oven in which a separation column is wound directly onto a tubular heat conducting support that contains a resistance heating element. This heating element is temperature programmed to provide the necessary thermal energy for the separation column. While this development reduced the size of the gas chromatograph, the power consumption continues to remain high at around 1 kW.
A different method of thermally modulating capillary separation columns arose in response to the need for reduced size and power consumption gas chromatographs. This method entails the coupling of thermally modulating elements, temperature sensing elements and capillary column material as a single unit. Once such method is described in U.S. Pat. No. 5,005,399 where a thin conductive film is deposited on the surface of a fused silica capillary column which is then wound onto an insulating support structure. A current is passed through the conductive film in order to thermally modulate the length of capillary column. This technique provided for a large reduction in power consumption versus convection oven designs but suffers from inconsistencies in the thin conductive film thickness, as well as damage to the thin conductive film during handling, coiling and heating/cooling cycles. These inconsistencies in film thickness along with damaged portions of the conduction film act to create variances in the resistance across the length of the capillary column which, when current is passed through the conductive film, causes localized hot spots to form on the capillary column surface. A very negative consequence of these hot spots is a non-uniform distribution of thermal energy across the length of the capillary column and ultimately an overall reduction in the chromatographic separating efficiency of the capillary column material. Also, when larger currents are passed through the thin conductive film to achieve faster temperature programming rates or to maintain the capillary column at higher temperatures, the localized hot spots begin to hasten the degradation of the thin conducting film which eventually leads to a failure of the heating system. In addition, the degree to which the capillary column can be compactly contained is limited by the insulation needed to protect successive coils from electrical short circuits, especially at higher temperatures above 200 degrees C., and the minimum bend radius that the thin conductive film can withstand before breakage occurs. This limitation usually results in a cylinder shaped arrangement of coils with a minimum radius of approximately 7.5 cm and cylinder height dependent on the length of capillary column needed. The final size of this column assembly quickly approaches that of a conventional wire-rack wound capillary column previously mentioned. Yet another drawback to using this method to thermally modulate the capillary column stems from the fact that the deposition of thin conductive films to the surface of fused silica capillary column material requires an intricate, detailed process that further complicates the manufacturing of the capillary column, thus severely limiting the variety of capillary column types available and manufactures willing to produce them. In the end, the use of thin film resistively heated capillary columns has major shortcomings when applied to the development of low power, small, fast gas chromatographs.
A second approach to the coupling of the capillary separation column, thermally modulating element, and temperature sensing element was developed by Roundbehler, et al. of Thermedics Detection Inc. in U.S. Pat. No. 5,808,178. This technique, which is marketed and sold under the trade names “Flash GC” and “EZ-Flash”, consists of a capillary separation column inserted into a conducting sheath (a metal tube in the commercially available version) or by direct resistive heating of a metal capillary separation column where the metal sheath or capillary column also operates as a resistance temperature detection device. While this technique solves the problem associated with the thin resistive film breakdown and hot spots of the previously discussed technique and consequently produces even thermal distribution across the length of the capillary column, the problem of having to electrically isolate successive coils from one another continues to limit the overall compactness of the design. Additionally, the extra thermal mass contributed by the resistively heated metal sheath in which the capillary column is placed, dramatically increases the power consumption of the device. It is stated in their patent that “a power supply capable of delivering up to 96 Volts at 12 Amps provides sufficient power to heat the tube (metal sheath) to desired temperatures” which is on the order of approximately 1 kW and approaching conventional convection oven power requirements, once again making it an impractical choice for the miniaturization of temperature programmed gas chromatographs.
Overton, et al. describe a miniaturized gas chromatograph in U.S. Pat. No. 5,611,846 which is sold by Analytical Specialists Inc. (ASI) under the trade name “microFAST GC” and uses a similar technique for thermal modulation of the capillary column as that of the previously discussed design. Instead of a metal sheath or metal capillary column used as the resistive heating element, a technique is described whereby a resistively heated wire is placed inside a small insulating sheath. This sheathed heater wire is then bundled in a parallel fashion with a low thermal mass, high resistance temperature sensing wire and a capillary separation column. The entire bundle is then placed inside a second insulating sheath and finally coiled onto an insulated support structure in a helix type geometry. The main advantage of this design lies primarily in the use of a wire arranged directly adjacent to and parallel with the separation column versus a relatively bulky metal sheath in which the capillary column is housed. The overall thermal mass of the assembly is considerably reduced which results in lowered power consumption and cool down times. This reduced level of power consumption allows for short 1-2 meter separation column assemblies to be practically implemented in a miniaturized gas chromatograph that is small, fast and relatively low in power consumption such as the “microFAST GC”. However, due to the extra steps involved with inserting the heater wire into a length of insulating sheath and then inserting the bundle of capillary column material, fragile sensor wire, and sheathed heater wire into a second insulating sheath, it becomes a very tedious, nearly impractical exercise to manufacture a separation column assembly in this manner any longer than 3 meters. This limitation in column length reduces the range of chemicals that can be separated in a single analysis and, while the power consumption is greatly reduced when compared to a conventional GC oven, in reality the column assembly still consumes enough power to prevent its use in truly portable, battery operated, temperature programmable GC designs.
In response to these shortcomings, Mustacich, et al., in U.S. Pat. No. 6,217,829, describe a reduced power consumption capillary separation column assembly that, in order to conserve energy, relies on a tightly packed geometry containing a resistively heated element, temperature sensing element and capillary separation column. The assembly is constructed by bundling all three elements in a parallel fashion then coiling the entire bundle into a tightly packed torus shaped geometry. The assembly is then wrapped in a thin layer of metal foil to prevent the assembly from uncoiling. The end result is a very low thermal mass, compact separation column assembly that can be rapidly heated and cooled. Claimed power consumption data is on the order of single digit Watts for temperature programming rates ranging from 1 to 10 degrees C./s to a final temperature of 180 degrees C. using capillary column lengths on the order of 1 meter. No mention is made of power consumption data for higher final temperature values or for longer separation column assemblies. Besides the complexity of manufacturing such an assembly, one drawback that this design presents deals with the round cross-sectional nature of a tightly packed, randomly arranged series of elements in a toroidal geometry. When the resistive element begins to heat the assembly, a temperature gradient almost immediately forms between the outer foil wrapped surface and the center of the cross-sectional area due to insulating effects that the outer coils have on the inner most coils. Because of the randomly arranged coils within the torus geometry, this results in uneven heating across the entire length of the capillary column and as a consequence, a reduction in the chromatographic separating efficiency of the capillary column material.
Capillary column separation assemblies that contain a thermally modulating element, a temperature sensing element and a capillary column as a single unit for integration with a gas chromatograph, as discussed in the preceding paragraphs, present another set of problems not yet discussed. In a conventional gas chromatograph where a convection oven is used to heat the capillary column, the oven is a permanent part of the device. Because of the numerous variables involved in the temperature feedback control system of the oven (e.g. variations in temperature sensing elements, variations in control electronics, variations in overall device calibration etc.), each gas chromatograph oven has its own unique heating characteristic or signature. When a capillary column needs to be replaced in a conventional GC the only factor affecting the performance of the device once the new capillary column is installed, is the new capillary column itself, not the GC oven since it remains a permanent part of the system. And, since capillary column manufacturing has become a nearly perfected art, the variations are usually small. However, with the capillary column assemblies that contain the thermally modulating element, temperature sensing element and capillary column as one unit, the entire oven and capillary column are being replaced at the same time therefore introducing a different heating characteristic to the overall system. Depending on the type of chemical analysis being performed, this could result in unacceptable performance variations after a capillary column assembly change is made. A second general problem associated with the various “single unit” capillary column assembly designs deals with the extra manufacturing burden generated from having to couple all of the various elements into a precise orientation and then the extra steps involved attaching them to insulated support structures and electrical lead connections. This usually involves having a person with expert technical skill make the column assemblies and ultimately a much higher end cost to the consumer.
Another technique that has been developed to thermally modulate a capillary separation column that claims low power consumption and compact design, uses microwave energy as its heating source and is described by Gaisford, et al. in U.S. Pat. No. 6,316,759. This technique involves the use of highly specialized resonant microwave cavities, a microwave source, a waveguide for channeling the microwaves to the resonant cavity and capillary columns specially coated with microwave absorbing material. While this technique may result in a minimal amount of power being needed to thermally modulate the separation column in a fairly compact form, the amount of specialized hardware necessary to perform this function make it an expensive, impractical choice for use in miniaturized portable/transportable temperature programmed gas chromatographs where ruggedness and simplicity of design are better suited.
One such example of a proposed rugged and simple design is described by Walte, et al. in U.S. Pat. No. 5,979,221. This design describes the use of an infra-red (IR) heat lamp as a heat source in which the heat lamp is positioned as a lid to an insulated open cavity. At the bottom or along the walls of this cavity resides either a cylindrical or flat spiraled capillary separation column. The heat lamp is not placed in intimate contact with the column assembly but rather heats by radiating thermal energy from a distance. This is most likely done in order to provide a more even thermal energy distribution from the heat lamp to the surface of the capillary separation column, however, this results in a much larger than necessary power requirement to heat the device. While there is nothing new and innovative about coiling a long thin object into a flat spiral, it is by no means a trivial task, due to the elastic, “springy” nature of capillary separation columns, to confine a length of capillary column material to a flat, spiraled orientation in which the final assembly is rugged and can withstand repeated exposure to temperatures above 300 degrees C. without structural failure. No mention is made in their description as to how such a rugged, flat, spiraled capillary column assembly could be constructed nor is there any mention of the chromatographic separating efficiency that could be obtained from the heat lamp/flat spiraled column combination.